Gas turbines both for electric generation and mechanical drive applications operate in a variety of environments. In order to adapt the machines to the environment where they have to operate and realize their full potential in terms of performance and reliability, it is necessary to treat the air that they consume. Although the dust concentration in the atmosphere is normally very low, a gas turbine requires such a large quantity of air to operate that even a medium sized heavy duty or aero-derivative gas turbine could ingest some hundreds of kilograms of dust per year if no efficient filtering systems are provided.
The efficiency of the axial compressor of a gas turbine is a direct function of the smoothness of the rotating and stationary blade surfaces and the shape of the airfoil profiles. These surfaces can be roughened by corrosion and erosion; while the ingestion of substances, which adhere to the surfaces (fouling) will modify their shapes. Furthermore, the ingestion of contaminants may generate high temperature corrosion, plugging of cooling passages and particle fusion on the high temperature sections of the gas turbine.
In order to prevent or limit damages which can be caused to the various turbo-machinery components by dust contained in the combustion air ingested by the compressor, a filter system is usually arranged upstream of the air inlet plenum of the gas turbine engine.
FIG. 1 illustrates a gas turbine system 100 comprising a gas turbine engine and a filter system according to the current art. The gas turbine system 100 comprises a gas turbine engine 103 and a combustion air delivery system 105. The gas turbine engine 103 usually comprises a gas generator including a compressor 107 and a high pressure gas turbine 109. Downstream of the high pressure gas turbine 109 a power turbine or low pressure turbine 111 is arranged, which produces useful power to drive a load (not shown). In some embodiments, typically in heavy duty gas turbines, a single shaft gas turbine is provided, which drives the compressor and provides useful power to the load.
Combustion air is fed through the combustion air delivery system 105 to an air inlet plenum 113 upstream from the compressor 107. The combustion air delivery system 105 comprises a filter system 115. The filter system 115 usually comprises a chamber 116 with an air inlet side 117 and an air outlet side 119. Within chamber 116 a filter arrangement 122 is provided, which separates the chamber 116 in the upper volume 116U and a downstream volume 116D. Air entering the chamber 116 through a plurality of vertically arranged hoods 121 flows through the upstream volume 116U and is filtered through the filter arrangement 122. The filter arrangement 122 is comprised of a plurality of filter cartridges 123 supported by a partition wall 125. The filtered air exiting the filter arrangement 122 flows through the downstream volume 116D towards a duct 127 and is delivered to the air inlet plenum 113 of the gas turbine engine 103.
The filter arrangement 122 is usually a so-called pulse filter arrangement or pulse-jet filter arrangement, such as the one described in EP 1086303. The filter cartridges 123 are periodically cleaned by pulse jets of compressed air oriented in a direction opposite the normal air flow direction through the filter system 122. Dirt accumulated on the upstream side of the cartridges 123 is detached therefrom and falls down to the bottom of the chamber 116 where it can be removed.
The air filters and filter arrangements is determined by the efficiency of a filter arrangement in separating particles from the air flow. Filters are usually classified into groups and classes. The classification is based on results of measurements on separation capacity obtained by means of standardized methods. A group is defined on the basis of a unified measurement method and the same test parameters. A class within a group is defined on the basis of one lower threshold value or two (upper and lower) threshold values of a filter separation capacity in relation to the method of the group whereto the class belongs.
The collection efficiency of the filter arrangement 122 is usually limited to a class F9 according to EN779:2012 standards or MERV 15 according to ASHRAE 52.2 standards.
In some situations, it would be desirable to have a more performing filtering arrangement by providing a further filter stage downstream of the filter arrangement 122, for example an EPA or HEPA filter. These highly performing filters are extremely expensive and, due to their collection efficiency, must be provided with efficient upstream coarse filtering arrangements, to prevent clogging thereof.
The use of EPA or HEPA highly performing filters downstream of a pulse jet filter stage 122, as the one shown in FIG. 1, is not commonly adopted, because when the filter cartridges 123 of the first filter stage 122 require replacement, dirt accumulated on the upstream side of the filter system 123, 125 is dragged through the partition wall 125 when individual cartridges 123 are removed. Dirt entrained or dragged by the high air speed flow generated through the apertures left open by the temporarily removed cartridges 123 contaminates the downstream volume 116D and the EPA or HEPA filter arrangement downstream thereof. To prevent these drawbacks, filter replacement should be carried out after turning off the gas turbine. Turbine shut down, on the other hand, causes serious economic losses.
There is therefore a need for a more efficient filter system which can be subject to maintenance while the gas turbine engine 103 is online, i.e. while the gas turbine engine is operating, eliminating or alleviating the abovementioned problems.